This invention relates to brushless excitation systems for synchronous dynamoelectric machines such as synchronous motors.
The type of system on which the present invention improves is represented by the following patents: Frola- U.S. Pat. No. 3,405,338; Hoffmann-U.S. Pat. No. 3,385,195; Hoffmann et al.- U.S. Pat. No. 3,414,788; Hoffmann: U.S. Pat. No. 3,470,435; Hoffmann- U.S. Pat. No. 3,748,555; Heyne- 3,845,369; and, Godwin et al. U.S. Pat. No. 3,959,702, the teachings of which are herein incorporated by reference.
The prior known system is generally characterized by including an exciter armature winding on a rotating assembly that also includes the synchronous motor field winding which is inductively coupled to the synchronous machine stator winding. Between the exciter armature winding and the motor field winding, and also part of the rotating assembly, are a rectifier and an excitation control system for selectively controlling the application of current therebetween. Representative excitation control systems are extensively described in the aforementioned patents and will not be detailed herein. In part, however, they include an excitation control switch that is directly connected between the rotating rectifier and the rotating field winding and this switch is to be turned on upon predetermined conditions. The excitation control switch is normally a three terminal semiconductor device such as a silicon controlled rectifier a thyristor which has its main anode and cathode terminals connected in the path between the rectifier and the field winding and its gate terminal connected to a gating circuit that applies a predetermined gating pulse for turn-on.
Such systems have been made and widely used with considerable success. While the general design of the control system permits various modifications for specific conditions to ensure that the gating pulse is of the proper character for reliable turn-on, it is desirable to permit the use of a standardized synchronization control system, packaged as a module, in a variety of conditions. This has been achieved in the past where the excitation voltage, applied to the exciter stationary winding, is DC. However, in instances where the same motor and excitation control system is for the application to adjustable frequency operation, such as having the ability to perform at various frequencies in a range of perhaps of 0 to 66 Hertz, there is an effect on the excitation control system that may impair reliable performance. This is a consequence if the motor field winding has an inherent inductance that is so high that the gate pulse from a standard synchronizing circuit module will not turn on the SCR. This is because the field current will not build up to the necessary holding value for the SCR during the length of the gate pulse. Instead, the field current goes out and the next gate pulse only repeats the process.
It is desirable to provide a simple and economical way to ensure that the excitation control SCR is turned on the first gate pulse. In doing so, it is preferred not to have to modify the gating circuit or other aspects of the synchronizing control circuit in any significant respect. The present invention accomplishes these purposes merely by providing a resistor, referred to herein as a holding current resistor, connected in parallel directly across the field winding. The holding current resistor is only of moderate size such as about 50 to about 150 ohms, typically about 100 ohms, for synchronous motor systems of principal interest. Since such systems are provided with a starting resistance or discharge resistance, such as resistor 68 of FIG. 1 of Frola U.S. Pat. No. 3,405,338, that is in a large number of parallel resistor strands, it is a simple matter either in manufacture or in the field to take one resistive strand of the starting resistor and reconnect it directly across the field winding as a holding current resistor. The magnitude and character of the holding current resistor is such that it does not impair to any significant degree the operation of the synchronous motor or result in undue losses. It is merely necessary to achieve a sufficient holding current of say, approximately 1 ampere, through the SCR to ensure its reliable turning on on a single gate pulse.
The inventive combination is useful generally in the excitation systems characterized by having a thyristor type excitation control switch. It is particularly useful where the excitation voltage is from a variable frequency source, as is sometimes used for energy conservation.